Surface-mounted device with leads

ABSTRACT

A surface-mounted device comprising a semiconductor chip ( 5 ) encapsulated in a package ( 1 ) and leads ( 2 ) projecting from the package so as to bring the semiconductor chip into contact with a substrate ( 3 ), which leads are provided at their surface, at the location of one of the pads provided for surface mounting, with a three-dimensional contact pattern.

The invention relates to a surface-mounted device comprising asemiconductor chip encapsulated in a package, and leads projecting fromthe package for bringing the semiconductor chip into contact with asubstrate. The invention relates, in particular, to a flat-packsurface-mounted device.

According to the state of the art, discrete semiconductor devices andintegrated circuits in the form of surface-mounted devices, customarilyabbreviated as SMD, are soldered directly onto the superficialinterconnect layer of substrates, for example printed circuit boards andceramic substrates.

To protect the semiconductor chips from undesirable mechanical andchemical influences and to conduct away and distribute dissipated heat,said semiconductor chips are customarily encapsulated in a package ofsynthetic resin, ceramic or metal. Only the external connections in theform of a plurality of projecting leads are led out of the package. Viathe leads the semiconductor chip encased in the package is connected,both electrically and mechanically, with the printed circuit board onwhich components are to be inserted.

Such surface-mounted devices with a package are customarily manufacturedsuch that a semiconductor chip is connected, for example by usingadhesive or solder, to a lead frame, i.e. a support comprising theleads. The pads of the semiconductor chip are subsequently connected tothe associated metallic lead frame surfaces by means of gold wires.

After said “wire bonding”, the semiconductor chip together with the goldwires and the adjoining regions of the lead frame are encapsulated usingan isolating molded compound, in such a manner that a so-termed“package” is formed from which the leads project laterally.

In the case of conventional packages, the end portion of the leads,which is not embedded in the package, is bent substantially parallel tothe bearing surface, so that, during assembly on a printed circuitboard, the end portion of the leads rests flat on the printed circuitboard and can be soldered there.

For the assembly of surface-mounted semiconductor elements use iscommonly made, particularly in automated production processes, ofso-termed SM-solder methods (surface-mount methods).

In this assembly process, the leads are initially provisionally attachedto solder connection spots on the printed circuit board, after whichthey are soldered in a special, fully automatic soldering process. SMsoldering processes include condensation soldering, infrared soldering,double-wave soldering, etc.

There are a large number of different, partly standardized packagestructures having different sizes and a different number of leads. Theyenable standardized manufacturing of printed circuit boards as well asautomatic component insertion on printed circuit boards.

These packages additionally have the advantage that a connecting leadarrangement led to the exterior by the chip pads enables improvedhandling of the chip in a subsequent mounting process.

Dependent upon the number of leads necessary, these packages correspondto a standard having specified dimensions so as to enable a standardizedproduction of printed circuit boards as well as automatic componentinsertion on printed circuit boards.

The dimensions of these packages are specified in German andinternational standards.

The reliability of an SMD soldered onto a substrate is substantiallyaffected if the thermal coefficient of expansion of the semiconductorelements differs appreciably from that of the substrate. Differentthermal expansion behavior of substrate and components causes high shearforces at the soldered connections, which may lead to cracks in thelatter at a later stage.

This problem is not as serious in the case of semiconductor elements ingull-wing packages since the leads then serve as flexible intermediatemembers between substrate and component, which are capable ofcompensating for stresses.

This problem is much more serious, however, in flat pack surface-mounteddevices.

Flat pack integrated chips are integrated circuits having a very flatstructure, wherein a carrier, customarily of a ceramic material, isprovided, besides the actual functional element (semiconductor chip),with conductor tracks by means of printing. For the external connectionuse is made of leads extending in a straight line or in slightly bentform from the package and being cut so as to form short connectionstubs. As a result, an electronic component having a very small heightis obtained.

In the case of the flat-pack design, the leads are arranged in oneplane, which is defined by the side of the package, which, afterassembly, faces the printed circuit board or extends parallel thereto.

These leads must be carefully designed to ensure good contact betweenthe semiconductor chip and the substrate accommodating said chip.

EP 0 468 420 discloses an integrated electronic component of flat-packdesign with laterally projecting leads, said leads having curled tips.

In the integrated electronic component disclosed in EP 0 468 420, theconstructional design of the leads is capable of dealing withthermomechanical stresses at the solder pads under certain conditionsonly.

Therefore it is an object of the present invention to provide asurface-mounted device comprising a semiconductor chip and leads,wherein the particular design of the pad of the leads precludes theoccurrence of cracks in a soldered connection.

In accordance with the invention, this object is achieved by asurface-mounted device comprising a semiconductor chip encapsulated in apackage and leads projecting from the package so as to bring thesemiconductor chip into contact with a substrate, which leads areprovided at their surface, at the location of one of the pads providedfor surface mounting, with a three-dimensional contact pattern.

This offers the advantage of an increase of the pad size and, as aresult of this surface modification, an improved adhesion between thepad of the leads and the solder.

In accordance with an embodiment of the invention, the three-dimensionalcontact pattern is provided with recesses.

An embodiment in which the recesses take the form of grooves or holes ispreferred.

The contact pattern may comprise recesses having sloping flanks.

In accordance with a particularly preferred embodiment, the contactpattern has recesses with sloping flanks whose mutual distance decreasesin the depth direction.

In accordance with a further embodiment of the invention, the recesseshaving sloping flanks are embodied so as to have a symmetrical sawtoothprofile. This embodiment satisfies, in particular, the requirementregarding a thermomechanically low-stress contact. Forces acting on thejoint between the leads and the solder are optimally distributed.

As a result of the toothed contact surface of the leads and the solder,movement of leads and substrate with respect to each other iseffectively precluded. One of the flanks of the saw teeth always takesup the thermo mechanical stresses. The tensile and shear forces at thepad are converted to compressive forces. As compressive forces do notlead to fatigue of the soldered connections, the service life of thelatter is substantially improved.

In accordance with a modification of the invention, thethree-dimensional contact pattern is arranged on the main surface of theleads facing the substrate.

In accordance with yet another modification of the invention, thethree-dimensional contact pattern is arranged on one of the lateralflank faces of the leads.

In said modification, the mechanical connection is made via the lateralsawtooth action, and the electrical connection is established by meansof the smooth main surface of the leads.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiment(s) described hereinafter.

In the drawings:

FIG. 1 is a cross-sectional view of a first embodiment of asurface-mounted device in accordance with the invention.

FIG. 2 is a side view of a first embodiment of a surface-mounted devicein accordance with the invention.

FIG. 3 is a diagrammatic plan view of a second embodiment of asurface-mounted device in accordance with the invention.

The invention relates to a surface-mounted device in a surface-mountedpackage (SMP) which is mounted directly on the surface of printedcircuit boards or ceramic substrates using SMT technology(surface-mounted technology). SMPs may be used on one side or on bothsides of printed circuit boards or in combination with customary leadedcomponents.

The invention particularly relates to a surface-mounted device with aFlat Pack and Quad Flat Pack (QFP) package. These are non-standardizedquadratic or rectangular plastic packages for integrated circuitscomprising up to more than 100 outward bent leads on two sides (FlatPack) or on all four sides (QFP).

The surface-mounted device comprises a semiconductor chip mounted on theisland of a lead frame, as well as leads connected to the bonding padsof the chip via bonding wires, and a package encapsulating the chip, theisland, the bonding wires and the internal end portions of the leads.

The semiconductor chip and the internal leads of the lead frame arefully encapsulated. Only the electrically contactable external endportions of the leads project from the package.

The package has an essentially flat lower side, and the term “lowerside” is to be understood to mean herein the surface of the package,which, after assembly on a substrate, faces the latter.

For the semiconductor chip (functional elements) use can be made, forexample, of the following non-mass produced component groups: discreteand integrated resistors, capacitors, inductors, sensors, ICs of alltypes, micro-acoustic and micro-optic ICs and other functional elements.Application takes place in all fields of electronic device technology.

In FIG. 1, reference numeral 1 denotes a package body and referencenumeral 2 denotes leads, which are arranged at the four circumferentialsurfaces of the package body 1 so as to project therefrom. The packagebody 1 is manufactured so as to be essentially flat by molding asynthetic resin material, the shape of the package body, in plan view,being quadratic or rectangular. The package body encapsulates theinternal end portions of the leads. The semiconductor chip 5 is attachedto an island 6.

In accordance with the invention, the leads have three-dimensionalcontact patterns at at least one of the pads. These three-dimensionalcontact patterns lead to a larger contact area with the solder and toimproved anchoring therein.

In accordance with an embodiment of the invention, the three-dimensionalcontact pattern has recesses. Such a contact pattern is for exampleobtained without a reduction in cross-section of the leads by anundulation of the leads.

An embodiment wherein the recesses take the form of grooves or holes ispreferred.

The contact pattern may comprise recesses with sloping flanks.

In accordance with a particularly preferred embodiment, the contactpattern comprises recesses having sloping flanks, such that the distancebetween said flanks decreases in the depth direction.

Such recesses are commonly referred to as notches.

Each notch has, for example, two essentially flat side faces and, viewedin cross-section, is for example approximately U or V-shaped. Thenotches extend perpendicularly across the outer pad of the leads and areopen at the two ends facing away from each other.

Viewed in cross-section, the notches may however alternatively be,partly or entirely, arc-shaped and, for example, at most orapproximately semi-circular. The term “U-shaped” is understood toinclude a polygonal shape.

In accordance with an embodiment of the invention, the three-dimensionalcontact pattern comprises in each case one notch for every lead. Inaccordance with a preferred embodiment, the three-dimensional contactpattern comprises in each case two notches for every lead.

The three-dimensional contact pattern may also be created by introducinglinear structures into the surface of the leads. Particularly suitableare recesses in the form of channels or indentations as they enablecomparatively large surfaces to be structured in a specific manner.These channels or indentations may extend parallel to each other.

The contact pattern may alternatively take the form of transverse ribsdefining a comb-like cross-section of the flank surface.

The notches may be arranged so as to include an angle in the range of 0to 90 degrees with the longitudinal axis of the lead. At 0 degrees, thenotches extend parallel to the longitudinal axis of the leads. A valuein the range around approximately 90 degrees is favorable. If the angleof intersection is chosen to be, for example, 45°, then a favorabletransfer of forces is achieved.

In a further embodiment of the invention, the recesses having slopingflanks are embodied so as to form a symmetrical sawtooth profile.

To enhance the resistance to extraction of the soldered connection, itproved advantageous to provide the notches with a sawtooth profile. As amatter of fact, the surface of the part provided with the sawtoothprofile is further increased and, as a result, its capability of beinganchored in the solder compound is optimized.

In accordance with a modification of the invention, the leads have acontact pattern at the main surface facing the substrate.

As shown in FIG. 3, a three-dimensional contact pattern, for example asawtooth profile, may be alternatively or additionally provided at atleast one of the lateral flank surfaces of the leads.

In this case, the part of the leads provided with the sawtooth-shapedflanks co-operates with the solder in such a manner that the projectionsof the leads captivate the leads in the soldered connection.

The position and size of the recesses of the contact pattern depend onthe size of the package, the power of the semiconductor element (currentdensity, frequency) and the spatial distribution within the package. Theposition of the contact pattern should be chosen such that the resultantanchoring of the leads precludes delamination at the solderedconnection.

Hereinafter, also the manufacture of the surface-mounted deviceaccording to the invention is elucidated by means of an example.

First of all a lead frame with a number of islands and associated leadsfor a number of semiconductor chips is manufactured.

The metallic lead frames which serve for the manufacture ofsemiconductor elements have a number of conductor tracks (leads)interconnected via stripes, to be removed at a later stage, and acarrier (island) for attaching the semiconductor chip.

The lead frame may be manufactured, in a two-stage etch process, from astarting body of metallic foil in the form of a strip. In the firststage, a unilateral or bilateral etch process is carried out in whichetching takes place across the entire thickness of the metal strip. As aresult, the desired basic shape of islands, leads and interconnectionstripes is obtained.

In the second stage, etching takes place superficially on one side onlyso as to form the desired three-dimensional contact pattern for theleads.

Various etching processes are known (dry, wet, isotropic, anisotropic,in plasma, chemical etc.) which are all suitable to generatecharacteristic patterns.

The lead frame may alternatively be manufactured, in a known andcustomary manner, by punching the starting body from a metal foilpresent in the form of a strip. Subsequently, as mentioned hereinabove,the details of the contact pattern of the leads and, optionally, of theislands are formed in an etch process.

In addition, there are various other methods of providing or removingthree-dimensional contact patterns using mechanical or chemical means.One mechanical surface treatment which deserves particular attention issand blasting. Sand blasting is an easy-to-use method which yields asatisfactory effect. The surface is substantially enlarged and, inaddition, a cramping effect takes place.

Patterning is also advantageously carried out using a laser. By virtuethereof, damage to the lead frame caused by mechanical loads isprecluded and a post-cleaning treatment following the patterningoperation can be dispensed with.

In a second processing step, the lead frame is bent in a pressing tool.In the bending process, the leads are bent twice and obtain the shapetypical of a flat pack package, as shown in FIG. 1.

In the next step, the chip is attached to the island of the lead frameby soldering or gluing.

Next, the bond pads of the chip are electrically connected to the innerpads of the leads by means of bonding wires.

Subsequently, the bonded semiconductor chip is packaged in an injectionmolding or molding process.

Said packaging operation is, for example, carried out as follows: thebonded semiconductor chip is introduced into an injection mold andsurrounded by an injection molding compound in an injection moldingprocess known as transfer molding. The leads of the component, whichproject, from the package have internal portions, which are embedded inthe package in a form-locking manner during encapsulating by injectionmolding.

After curing of the molded or injection molded compound, the externalportions of the leads are punched out, clipped and cleaned.

In accordance with a modification of the manufacturing process, thethree-dimensional contact pattern is generated by imprinting notches inthe leads after the encapsulation of the semiconductor chip.

The surface-mounted device thus manufactured has a flat lower side. Theexternal pads of the leads extend in the same plane as the lower surfaceof the package.

To process the surface-mounted devices, any of the known SMDtechnologies may be employed. The leads of these devices are permanentlyattached to rigid or flexible substrates by soldering, or optionally bybonding or gluing.

For the soldering process use can be made of a reflow method (reflowsoldering) or of other known methods such as, for example, wavesoldering using a tin bath.

In the insertion process, the surface-mounted device is provided on theprinted circuit board in such a manner that the leads are situated abovethe solder-coated pads of the printed circuit board. The pads having athree-dimensional contact pattern, which extend parallel to the bearingsurface, directly face the solder-coated pads of the printed circuitboard. Next, the printed circuit board is fed, in known manner, into areflow solder oven in which the solder is melted and the leads' padshaving a three-dimensional contact pattern are soldered to the pads ofthe printed circuit board. As shown in FIG. 2, the pads of the leads 2facing the solder are thus connected to the solder 4 in a form-lockingmanner. By virtue thereof, a durability of the mechanical and electricalconnection between semiconductor component and substrate (printedcircuit board) 3 under temperature interaction conditions is achievedwhich would otherwise be impossible.

1. A surface-mounted device comprising a semiconductor chip encapsulatedin a package and leads projecting from the package so as to bring thesemiconductor chip into contact with a substrate which leads areprovided at their surface, at the location of one of the pads providedfor surface mounting, with a three-dimensional contact pattern.
 2. Asurface-mounted device as claimed in claim 1, characterized in that thethree-dimensional contact pattern is provided with recesses.
 3. Asurface-mounted device as claimed in claim 1, characterized in that thethree-dimensional contact pattern has recesses in the form of grooves orholes.
 4. A surface-mounted device as claimed in claim 1, characterizedin that the three-dimensional contact pattern has recesses with slopingflanks.
 5. A surface-mounted device as claimed in claim 1, characterizedin that the three-dimensional contact pattern has recesses with slopingflanks whose mutual distance decreases in the depth direction.
 6. Asurface-mounted device as claimed in claim 1, characterized in that thethree-dimensional contact pattern has recesses with a symmetricalsawtooth profile.
 7. A surface-mounted device as claimed in claim 1,characterized in that the three-dimensional contact pattern is arrangedon the main surface of the leads facing the substrate.
 8. Asurface-mounted device as claimed in claim 1, characterized in that thethree-dimensional contact pattern is arranged on one of the lateralflank faces of the leads.